The long-term goal of this research is to identify the specific mechanisms involved in the extreme emotional reactivity that characterizes Borderline Personality Disorder (BPD) and to apply these findings to the development and validation of both psychosocial and psychopharmacological treatments that directly target these mechanisms. The specific aim of the proposed research is to identify the characteristic patterns of physiological and phenomenological reactions in response to social perturbation in individuals with BPD as compared to two healthy comparison groups: 1) a temperamentally matched comparison group (scoring similarly to BPD patients on trait measures of negative affect and impulsivity);and 2) a non-temperamentally matched comparison group (scoring in the average range on these measures). All participants will be assessed for DSM-IV Axis I and II disorders with structured interviews and then will participate in a laboratory stress paradigm involving a speech and a verbal arithmetic task in front of three judges. Saliva samples will be collected at several time points in order to examine two physiological dimensions of the stress response: 1) salivary cortisol, reflecting reactivity of the limbic hypothalamic-pituitary-adrenal axis;and 2) alpha-amylase, reflecting activation of the autonomic (primarily sympathetic) nervous system. It is predicted that in response to the laboratory stress paradigm: 1) BPD patients, compared to both comparison groups, will show increased cortisol and alpha-amylase responses and slower return to baseline levels;and 2) BPD patients, compared to both comparison groups, will report greater subjective distress. It is also predicted that, compared to both comparison groups, BPD patients will report more malevolent perceptions of those with whom they have interacted during the experimental procedures. This research is important to public health because BPD is a highly prevalent, costly, painful, debilitating, and deadly disorder, and thus represents a serious clinical and public health concern. For individuals with BPD, subtle and benign events in the environment can evoke intense emotional responses that activate the stress response system and may rapidly spiral into behaviors that are life threatening and functionally debilitating. There are also serious health consequences to chronic elicitation of the stress response, and individuals with BPD are frequent users of both medical and mental health services. Understanding the physiological and psychological processes underlying these problematic features of BPD is imperative to the development of treatments aimed at reducing the burden of mental illness.